1. Field of the Invention
The present invention relates to an automobile headrest, and more particularly, to an automobile headrest having a height adjusting mechanism and a falloff preventing mechanism for preventing the headrest from easily falling off from a seat back.
2. Description of the Related Art
It has been a common practice to mount a headrest to an automobile seat for the purpose of promoting safety and protecting a passenger in case of an accident such as a collision. To be adjustable to an arbitrary vertical position according to a physical constitution or preference of a passenger, the headrest is provided with a height adjusting mechanism. In general, two headrest stays are firmly fixed to the headrest and supported by being inserted respectively in two headrest bushes that are securely installed on a side of a seat back. In addition, one of the headrest bushes is provided with the height adjusting mechanism, and the other headrest bush is free.
In the case of a collision (including a rear-end collision), the automobile headrest prevents the head of a passenger in the seat from being tilted backwards too much due to an impact of the collision. At this moment, an impact force applied to the head of the passenger acts as a load pressing the headrest downwards. Therefore, the height adjusting mechanism of the automobile headrest allows the headrest to move upwards upon application of an extracting force thereto and to stop at an arbitrary height, and locks the headrest to prohibit it from moving downwards unless an unlocking operation is performed (e.g., see JP 04-193109 A).
To be more specific, the headrest stays of the headrest are vertically adjustably inserted and supported in the headrest bushes that are securely installed on the side of a seat back. Lock members that reciprocate owing to a resilient force are attached to the headrest bushes at positions corresponding to an upper end portion of the seat back and abut on the headrest stays, respectively. The headrest stays have a plurality of height adjusting notches arranged at a predetermined spacing. The headrest is held at a predetermined height through engagement of the lock members with the notches. In this state, the height adjusting notches are so shaped as to allow the headrest stays to move upwards in relation to the lock members but to prohibit the headrest stays from moving downwards.
FIGS. 7 and 8 are each a partially broken exploded perspective view and a sectional perspective view of a main part of an example of the lock member. A headrest stay 3 of a headrest is inserted and supported in a tubular headrest bush 6, and a height adjusting mechanism 7 is attached to the headrest bush 6 at a position corresponding to an upper end portion of a seat back. In the height adjusting mechanism 7, a lock plate (lock member) 8 is attached to the headrest bush 6 so as to be free to reciprocate perpendicularly to the headrest stay 3, which penetrates the lock plate 8. A portion 8a of the lock plate 8 is pressed against the penetrating headrest stay 3 due to a resilient force of a spring 12. In practice, the lock plate 8 and the spring 12 are accommodated in a case 13, and only an operating portion 8b of the lock plate 8 protrudes outside. The headrest stay 3 has a plurality of height adjusting notches 4 arranged at a predetermined spacing. The lock plate 8 engages one of the notches 4 owing to the resilient force of a spring 12, to thereby lock the headrest stay 3. As shown in FIGS. 7 and 8, each of the height adjusting notches 4 is formed of a horizontal face 4a that extends toward the center of the headrest stay 3 and an inclined face 4b that extends downwards from the deepest portion of the horizontal face 4a. The inclined face 4b is gradually increased in diameter with distance from the deepest portion of the horizontal face 4a. 
With this construction, when the headrest is pulled (drawn out) upwards, the portion 8a at which the lock plate 8 abuts on the headrest stay 3 is released from the notch 4 against the resilient force (spring force) of the spring 12 by the action of the inclined face 4b of the notch 4. As a result, the headrest stay 3 can be moved upwards and the height of the headrest can be adjusted. On the contrary, when the headrest is pressed downwards, the lock plate 8 remains engaged with the horizontal face 4a of the notch 4 without being released therefrom. Therefore, the headrest stay 3 is allowed to move downwards when the operating portion 8b of the lock plate 8 is pressed against the spring 12 and the lock plate 8 is released from the notch 4. Accordingly, the downward movement of the headrest can be adjusted.
As is apparent from the foregoing construction, the headrest equipped with the conventional height adjusting mechanism as described above can be easily drawn out from the seat. Therefore, the headrest may be drawn out inadvertently or on purpose, which raises a problem in that the safety of the passenger cannot be guaranteed in case of an accident such as a collision. Further, such the headrest cannot meet the requirements of the American National Standards FMVSS 202.
Thus, a headrest provided with a headrest falloff preventing mechanism is disclosed as a solution to the problem and as a product meeting the requirements of the American National Standards FMVSS 202 (e.g., see JP 2003-259935 A).
However, the conventional headrest falloff preventing mechanism can prevent the headrest from falling off, there is a problem in that the headrest cannot be drawn out from the seat. In practice, however, it is often necessary to draw the headrest out from the seat for the purposes of cleaning, maintenance, and the like. Further, in the case where an accident occurs when the seat is reclined backwards, the headrest tends to spring out from the seat due to a force acting on the headrest, but even in such the case, the falloff prevention needs to be reliably effected. According to the conventional example described above, for example, in such the case, speed with which the headrest stays come off is high, so the lock members pass over engagement recesses before engaging with the engagement recesses. In other words, there is another problem in that the falloff prevention does not work, so the headrest falls off.